To measure the earth resistance

To measure the earth resistance.

Aim:

To measure the earth resistance.

Apparatus required

Theory

All the electrical installations and appliances should be earthed properly for ensuring human safety. A separate wire, known as earth wire runs along the supply line and is connected to the ground through an earth electrode. The total resistance of the earthing system should be small so that in the event of any fault, the fault current is sufficiently high to blow off the fuse. The earth resistance is the resistance offered by the soil and the electrode to the flow of earth leakage current, which will flow in case of earth fault only. The earth tester is a special type of ohmmeter which sends ac through earth and dc through the measuring instruments as shown in Fig. The direction of flow of current in the ground keeps on alternating due to current reverse whereas current directions in the two reverser and potential reverser are mounted on the main shaft of hand driven dc generator. The working principle of an earth tester is identical to that of megger. There are two moving coil viz. potential and current coil, which are deflected in the magnetic field of a permanent magnet. The hand driven generator or a set of batteries supply power to these coils. It has four terminals P₁, E₁, P₂ and E₂. Terminals P₁ and E₁ are shorted to form a common point which is connected to the earth electrode under test. The other two terminals E₂ and P₂ are connected to the auxiliary electrode A and B respectively. The value of earth resistance is indicated directly on the scale when the test button is pressed. The value of earth resistance depends upon the soil condition and its moisture contents. In hilly areas the earth resistance is higher if electrodes are not place properly in contact with the earth. Water content in the soil decreased the earth resistance. The normal value of earth resistance should lie between 1 to 2 Ω.

Procedure

1. Connect the earth tester as shown in Fig.

2. Switch ON the earth tester

3. Adjust the resistance range button between 10Ω to 1000Ω and set to 10Ω.

4. Change the position of electrode B by 1m on the either side and observe the earth resistance by pressing the test button.

Observation table

Conclusion

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